1. Field of the Invention
The invention relates to homobimetallic and heterobimetallic alkylidene complexes of ruthenium containing N-heterocyclic carbene ligands and a process for preparing olefins from acyclic olefins having two or more carbon atoms or/and from cyclic olefins having three or more carbon atoms by olefin metathesis, in which at least one of these bimetallic alkylidene complexes is used as catalyst.
2. Description of the Prior Art
Transition metal-catalyzed formation of Cxe2x80x94C bonds is among the most important reactions of organic synthetic chemistry. Olefin metathesis is an important example of such a reaction since it enables olefins free of by-products to be synthesized. Olefin metathesis has not only a high potential in the field of preparative, organic synthesis, e.g. for ring-closure metathesis (RCM), ethenolysis or the metathesis of acyclic olefins, but also in polymer chemistry, e.g. for ring-opening metathesis polymerization (ROMP), acyclic diene metathesis (ADMET) or alkyne polymerization.
Since its discovery in the 1950s, a number of industrial processes have been able to be realized. Nevertheless, olefin metathesis has advanced to provide a broadly applicable synthetic method only in recent times as a result of the development of new catalysts (for a review article, see: J. C. Mol in: B. Cornils, W. A. Herrmann, Applied Homogeneous Catalysis with Organometallic Compounds, VCH, Weinheim, 1996, p.318-332; M. Schuster, S. Blechert, Angew. Chem. 1997, 109, 2124-2144; Angew. Chem. Int. Ed. Engl. 1997, 36, 2036-2056; R. H. Grubbs, S. Chang, Tetrahedron 1998, 54, 4413-4450).
Numerous, fundamental studies have contributed significantly to an understanding of this transition metal-catalyzed reaction in which an exchange of alkylidene units between olefins occurs. The generally accepted mechanism involves metal-alkylidene complexes as active species. These react with olefins to form metallacyclobutane intermediates which undergo cycloreversion to again generate olefins and alkylidene complexes. The isolation of metathesis-active alkylidene and metallacyclobutane complexes supports these mechanistic hypotheses.
Numerous examples are found, in particular, in the complex chemistry of molybdenum and tungsten. The work of Schrock, in particular, has revealed well-defined alkylidene complexes whose reactivity can be controlled (J. S. Murdzek, R. R. Schrock, Organometallics 1987, 6, 1373-1374). The introduction of a chiral ligand sphere into these complexes makes it possible to synthesize polymers having a high tacticity (K. M. Totland, T. J. Boyd, G. C. Lavoie, W. M. Davis, R. R. Schrock, Macromolecules 1996, 29, 6114-6125). Chiral complexes of the same structural type have also been used successfully in ring-closure metathesis (O. Fujimura, F. J. d. I. Mata, R. H. Grubbs, Organometallics 1996, 15, 1865-1871; J. B. Alexander, D. S. La, D. R. Cefalo, A. H. Hoveyda, R. R. Schrock, J. Am. Chem. Soc. 1998, 120, 4041-4042). However, the high sensitivity to functional groups, air and water is a disadvantage.
Recently, phosphine-containing complexes of ruthenium have become established (R. H. Grubbs, S. T. Nguyen, L. K. Johnson, M. A. Hillmyer, G. C. Fu, WO 96/04289, 1994; P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs, Angew. Chem., 1995, 107, 2179-2181; Angew. Chem. Int. Ed. Engl. 1995, 34, 2039-2041; R. H. Grubbs, E. L. Dias, Organometallics, 1998, 17, 2758). Owing to the electron-rich, xe2x80x9csoftxe2x80x9d character of the later transition metals, these complexes have a high tolerance toward hard functional groups. This is demonstrated, for example, by their use in natural product chemistry (RCM of dienes) (Z. Yang, Y. He, D. Vourloumis, H. Vallberg, K. C. Nicolaou, Angew. Chem. 1997, 109, 170-172; Angew. Chem., Int. Ed. Engl. 1997, 36, 166-168; D. Meng, P. Bertinato, A. Balog, D. S. Su, T. Kamenecka, E. J. Sorensen, S. J. Danishefsky, J. Am. Chem. Soc. 1997, 119, 2733-2734; D. Schinzer, A. Limberg, A. Bauer, O. M. Bxc3x6hm, M. Cordes, Angew. Chem. 1997, 109, 543-544; Angew. Chem., Int. Ed. Engl. 1997, 36, 523-524; A. Fxc3xcrstner, K. Langemann, J. Am. Chem. Soc. 1997, 119, 9130-9136).
However, the opportunities for varying the phospine ligands used are greatly limited due to steric and electronic factors. Only strongly basic, bulky alkylphosphines such as tricyclohexylphosphine, triisopropylphosphine and tricyclopentylphosphine are suitable for the metathesis of acyclic olefins and relatively unstrained ring systems. Accordingly, the reactivity of these catalysts cannot be adjusted. Furthermore, chiral complexes of this structural type cannot be obtained.
It has already been able to be shown by the inventors of the present invention that the introduction of N-heterocyclic carbenes as ligands not only enables the activity of these systems to be increased but also makes it possible to achieve novel control possibilities, e.g. in respect of chirality, tacticity or regulation of the activity, owing to the significantly more variable ligand sphere (T. Weskamp, W. C. Schattenmann, M. Spiegler, W. A. Herrmann, Angew. Chem. 1998, 110, 2631-2633; Angew. Chem. Int. Ed. Engl. 1998, 37, 2490-2493).
However, even when they have tolerance toward functional groups, all ruthenium systems still have an activity which is significantly less than that of molybdenum and tungsten.
For these reasons, it is an object of the invention to develop tailored metathesis catalysts which have not only a high tolerance toward functional groups and a more variable ligand sphere but also significantly increased activities.
This object is achieved according to the invention by a complex of ruthenium having the structural formula I, 
where
X is an anionic ligand,
Z is a monodentate to tridentate ligand which contains a metal and is nonionically bound to the ruthenium center,
R1 and R2 are identical or different and can also form a ring,
R1 and R2 are each hydrogen or/and a hydrocarbon group, where the hydrocarbon groups can be identical or different and each be a straight-chain or branched or/and cyclic radical selected from the group consisting of alkyl radicals having from 1 to 50 carbon atoms, alkenyl radicals having from 2 to 50 carbon atoms, alkynyl radicals having from 2 to 50 carbon atoms, aryl radicals having from 6 to 30 carbon atoms and silyl radicals, where one or more of the hydrogen atoms in the hydrocarbon or/and silyl groups may be replaced by identical or independently different alkyl, aryl, alkenyl, alkynyl, metallocenyl, halogen, nitro, nitroso, hydroxy, alkoxy, aryloxy, amino, amido, carboxyl, carbonyl, thio or/and sulfonyl groups, the ligand L is an N-heterocyclic carbene having one of the formulae II-V, 
where R1, R2, R3 and R4 in the formulae II, III, IV and V are identical or different and are each hydrogen or/and a hydrocarbon group, where the hydrocarbon groups are identical or different and are independently cyclic, straight-chain or/and branched radicals selected from the group consisting of alkyl radicals having from 1 to 50 carbon atoms, alkenyl radicals having from 2 to 50 carbon atoms, alkynyl radicals having from 2 to 50 carbon atoms, aryl radicals having from 6 to 30 carbon atoms, in which at least one hydrogen may be replaced by a functional group and where R3 and R4 may be identical or different and may each independently be a halogen, nitro, nitroso, alkoxy, aryloxy, amido, carboxyl, carbonyl, thio, silyl or/and sulfonyl group.